1. Field of the Invention
This invention relates to catheters and catheter systems. In particular this invention relates to seals in catheters, particularly to angioplasty catheters.
2. Background Description
At present, there exist four distinct functional classes of dilatation balloon catheters/catheter systems: (1) "over-the-wire" catheter systems, (2) "semi-movable" catheter systems, (3) "fixed-wire" catheter systems, and (4) "balloon-on-a-wire" devices. "Over-the-wire" catheter systems permit full rotational and full coaxial mobility of the guidewire relative to the catheter component of the system. "Over-the-wire" catheter systems are the only dilatation balloon angioplasty systems of the prior art that permit separation of the catheter component from the guidewire component. "Over-the-wire" catheters can be fully withdrawn over a guidewire, and they will accept the antegrade and retrograde introduction of a guidewire therethrough. U.S. Pat. No. 4,323,071 describes an "over-the-wire" system. "Semi-movable" catheter systems permit full rotational and limited coaxial mobility of the guidewire installed therein. U.S. Pat. No. 4,616,653 describes a "semi-movable" system. "Fixed-wire" catheter systems permit limited rotation mobility and no coaxial mobility of the guidewire relative to the catheter component of the system. U.S. Pat. No. 4,582,181 describes a "fixed-wire" system. "Balloon-on-a-wire" devices do not provide any mobility of the guidewire relative to the balloon component of the system. Rotation of the directional guidewire disposed at the distal aspect of these devices requires rotation of the entire device. International Patent Application No. PCT/US86/00938 entitled "Microdilatation Probe and System for Performing Angioplasty" describes a "balloon-on-a-wire" device.
"Over-the-wire" systems were the first systems introduced that provided satisfactory "steerability." FIGS. 2A-2E have been included in this application to illustrate the design features fundamental to "over-the-wire" systems currently marketed worldwide. Importantly, note that the shaft of the catheter 40 contains two separate lumens 42,46 which function independently to accommodate a guidewire 80 and to transmit fluid and hydraulic pressure along the length of the catheter. Occasionally, these catheters are constructed with three shaft lumens with the third functioning as a vent for the balloon. The proximal end 30 of the catheter contains an adapter 32 designed to interface with a source of hydraulic pressure, while the distal end 50 contains a "balloon-like" structure 52 for dilation of the artery or other vessel. In the case of the catheter illustrated in FIGS. 2A-2E, the catheter is composed of two pieces of concentric tubing. Although alternative designs exist for the construction of multi-lumen dilatation balloon catheters, it should be recognized that the fundamental features of all prior art "over-the-wire" catheters are similar. For the purpose of simplicity, we will confine our remarks concerning "over-the-wire" catheters to the catheter configuration depicted in FIGS. 2A-2E, with the understanding that these remarks apply to all prior art "over-the-wire" devices, regardless of configuration.
The design features that distinguish "semi-movable" systems from "over-the-wire" systems concern: (1) the profile of the segment of the guidewire contained within the catheter component of the device and (2) the permanence of the guidewire within the confines of the catheter. The guidewires contained within these devices are significantly lower in profile relative to the profiles of stand alone guidewires used in conjunction with "over-the-wire" systems. In fact, the guidewires contained within these devices have been rendered sufficiently low in profile and thus sufficiently delicate that they cannot tolerate the abuse that stand-alone guidewires commonly receive during the course of a routine angioplasty procedure. As a result, "semi-movable" devices have been designed such that the guidewires cannot be removed from the protective confines of the catheter shaft.
A "fixed" guidewire/catheter composite system consists of a single-lumen dilatation balloon catheter that contains a low profile guidewire which extends therethrough. The catheter and guidewire are immovably bonded together at the distal aspect of the balloon component of the catheter. These devices are commonly manufactured with a "torque limiter" that functions to limit the rotational mobility of the guidewire relative to the catheter component of these systems. The guidewire provides column strength for the balloon.
A "balloon-on-a-wire" system, as the name implies, consists of a guidewire that contains a lumen which communicates with a balloon disposed on the shaft of the guidewire. The balloon is immovably bonded to the guidewire.
The balloon component 52 of a conventional dilatation catheter usually is referred to as a "balloon-like" structure because the balloon component is not a true balloon. The "balloon-like" segment consists of an aneurism within a segment of the tubing 48, that has a lumen that extends therethrough. In the case of "over-the-wire" and "semi-movable" systems, the hydraulic competence of the "balloon component" is achieved by bonding the inner surface of tubing 48 to the outer surface of a length of tubing 44 contained therein at the distal aspect of the device 51. In the case of "fixed-wire" and "balloon-on-a-wire" systems, hydraulic competence is achieved by bonding the balloon component onto the guidewire that extends therethrough.
Considerable effort has been directed toward the development of dilatation balloon catheters with progressively smaller cross-sectional profiles. Catheters with lower profiles provide several advantages. They provoke less trauma during the process of introduction within the intra-vascular system. They require less effort to manipulate across severe obstructions, and they create less impairment to blood flow within the vessel containing the device (as illustrated in FIG. 2E).
A small profile also permits superior intraoperative angiographic delineation of the vascular system relative to larger profile catheters. Angiograms must be obtained intermittently during the course of a routine angioplasty procedure to reassess the vascular anatomy. This is generally accomplished by injecting contrast media into the vascular system via the guiding catheter that contains the dilatation balloon catheter. (Contrast media is a radio-opaque liquid that functions to opacify the vasculature when injected during the performance of an arteriogram.) The resolution of conventional angiography is directly related to the rate of contrast injection, which, in turn, is directly related to the cross-sectional area of the channel used to convey the contrast media into the intra-vascular system. During the course of an angioplasty, the cross-sectional area of this channel effectively equals the cross-sectional area of the lumen of the guiding catheter minus the corresponding cross-sectional area of the shaft of the dilatation catheter contained therein. The use of dilatation catheters with lower shaft profiles thus permits the delivery of higher flow rates of contrast fluid to permit the performance of intra-operative angiography with enhanced resolution.
In short, lower profile catheters are easier and safer to use relative to larger profile counterparts. Their use provokes less trauma, both during introduction into the vascular system and during manipulation across an intra-vascular stenosis. Low profile devices further provoke less impairment of blood flow and permit superior angiographic definition of the vascular anatomy relative to larger profile devices. Hence, considerable effort has been devoted to their development.
Although considerable progress has evolved in this regard, it should be recognized that much of the progress concerning "over-the-wire" systems has resulted from the process of miniaturization. The use of state-of-the-art plastics has permitted the manufacture of these devices with progressively thinner walls. The development of new technologies has permitted the construction of these devices with progressively smaller caliber channels. Relatively little benefit has resulted from efforts to modify the fundamental "over-the-wire" catheter design, which has remained essentially unchanged since it was introduced.
Despite the progress that has been achieved as a result of miniaturization, this practice has provided gradually diminishing returns. In addition, it has been responsible for the development of several functional limitations. This process accounts for the fact that current generation low profile "over-the-wire" catheter systems have (1) compromised "pushability," (2) compromised balloon inflation/deflation rates, (3) limited guidewire compatibility, (4) compromised guidewire torque transmission, (5) compromised trackability, (6) enhanced propensity for air entrapment, (7) enhanced balloon fragility, and (8) lowered balloon profiles, relative to larger profile devices.
With respect to the first issue, the "pushability," or column strength, of a catheter varies directly with the rigidity of the plastic that is used in the construction of the shaft as well as the thickness of the walls used in the construction of the catheter shaft. Conventional low profile systems tend to have inferior "pushability" relative to larger profile systems because the channel walls of these devices tend to be thinner. As a result, low profile devices are prone to axial compression and buckling during introduction within critical stenoses, two features that limit their utility in the treatment of high grade lesions. This circumstance has been partially offset by the use of more rigid plastics in the construction of these devices.
With respect to the second issue, prolonged inflation/deflation times, the rate of inflation and deflation of the "balloon-like" component of a dilatation balloon catheter is directly related to the cross-sectional area of the hydraulic channel 46 that communicates with the balloon and conveys the hydraulic pressure along the length of the catheter, and to the viscosity of the hydraulic medium that is used to transmit the hydraulic pressure. Because the channels contained within low profile systems of the prior art tend to be smaller relative to prior generation catheters, the corresponding inflation and deflation times for these devices tend to be longer. This feature enhances the risk of provoking ischemically-mediated complications with the use of these devices. This circumstance derives from the fact that partial balloon inflation compromises blood flow and accomplishes no therapeutic benefit. Balloons are partially inflated during the process of balloon inflation and deflation. Again, this adverse feature has been partially offset by the use of more rigid plastics in the construction of the catheter shaft. This practice has permitted the manufacture of the shafts with thinner walls and hence larger lumens relative to previous generation devices.
With respect to the third issue of guidewire compatibility, the larger guidewires (e.g., 0.018 inch diameter) are preferred by most operators for procedures involving critical stenoses, the very lesions for which the low profile catheters were developed. This preference arises because the larger profile guidewires offer greater stability, offer greater structural integrity, transmit greater torque, tolerate greater force, and permit superior steerability relative to smaller profile guidewires. In short, these wires can be negotiated within the confines of complex or high grade lesions more reliably and more consistently relative to lower profile wires. Because the guidewire channels of low profile systems tend to be small, they frequently cannot accommodate the large profile wires. Hence, the use of low profile catheters frequently obligates the concomitant use of low profile guidewires, a requirement that predisposes the patient receiving the procedure to enhanced morbidity relative to the optimal circumstance.
With respect to the fourth issue, compromised guidewire torque transmission, the efficiency with which torque is transmitted via the guidewire is inversely related to the friction generated by rotation of the guidewire within the confines of the guidewire catheter channel. Torque transmission is required to direct the guidewire within the patient's vasculature. Clearly, the amount of friction that develops in response to guidewire rotation is a function of the extent to which the outside surface of the guidewire contacts the luminal surface of the guidewire channel and to the coefficient of friction of the guidewire, catheter interface. And the propensity for guidewires to come in contact with the luminal surface of catheters increases as the profile of the guidewire channel is reduced. Because low profile catheters contain smaller guidewire channels, they provide diminished guidewire torque delivery and hence diminished directional control, relative to larger profile devices. This circumstance has been partially offset by the application of a lubricous coating to the guidewires contained within these systems.
With respect to the fifth issue, compromised catheter "trackability," the ease with which a catheter courses over a guidewire varies as a function of: (1) the resistance generated between the catheter and guidewire during coaxial movement of the catheter relative to the guidewire, and (2) the flexibility of the catheter component of the system. Hence, the "trackability" of a catheter relates in part to the size of the guidewire channel. Because lower profile catheters contain lower profile guidewire channels relative to the larger profile devices, these devices provide less "trackability" relative to the larger profile devices. This circumstance has been partially offset by: the manufacture of these devices with thinner walls and hence more flexible catheter shafts, and the use of lubricous coatings on the surfaces common to the guidewire and guidewire catheter channels of these devices.
With respect to the sixth issue, air entrapment, the air must be evacuated from the hydraulic channel of all dilatation balloon catheters before these devices can be introduced into the vascular system. Failure to evacuate the air contained within this channel predisposes the patient receiving the procedure to the risk of an air embolism in the event of a balloon rupture. In addition, air contained within the hydraulic channel compromises the hydraulic function of these devices. In general, the hydraulic channel of a conventional "over-the-wire" catheter is sealed at the distal end. Preparation of this channel requires a two-step process. First, the air contained within the hydraulic channel must be evacuated. This is generally accomplished by applying a syringe to the proximal end of this channel and aspirating the contents. Next, the channel must be filled with dilute contrast medium (the universal hydraulic fluid used in conjunction with dilatation balloon catheters). The introduction of contrast media in the setting of incomplete evacuation results in the entrapment of air within the distal aspect of the hydraulic channel (e.g., within the confines of the balloon) and the creation of an air bubble.
The ease with which an air bubble can be removed from the balloon following the introduction of contrast relates directly to the dimensions of the hydraulic channel. It is substantially more difficult to remove air bubbles from smaller profile channels relative to larger profile channels. As a result, "miniaturized" low profile catheters with low profile hydraulic channels are prone to air entrapment. This circumstance has been offset by the development of a variety of air vents that can be installed in the distal aspects of these channels. The function of these vents takes advantage of the fact that the viscosity of a fluid consistently exceeds the corresponding property of a gas.
One example of a vented "over-the-wire" catheter is described in U.S. Pat. No. 4,638,805. In this example, a small passageway is provided from the lumen of the balloon to the tip of the catheter. The passageway is formed by placing a very small diameter wire between the distal aspect of the balloon and the central column within the balloon that contains the guidewire. When the catheter is manufactured, the balloon and the central column are heat-shrunk together with the wire in place. The wire is later removed to allow a small passageway for the exit of air from the balloon during filling of the balloon. By making the passageway sufficiently small, fluid may be retained within the balloon while air is expelled therefrom.
U.S. Pat. No. 4,811,737 describes an alternative method for venting an "over-the-wire" catheter. This patent describes a catheter with a small slit in the exterior surface of the balloon. When fluid is introduced into the balloon, air is forced out of the small slit. The inflation fluid is sufficiently viscous to prevent its escape through the same slit. Unfortunately, the slit creates a region in the surface of the balloon which is prone to failure. When the balloon is inflated to pressures of many atmospheres, the stresses are concentrated at the ends of the slit, making it prone to rupture.
U.S. Pat. No. 4,821,722 describes the use of micro-machined openings in the central shaft and balloon surface for the purpose of selectively venting air from the hydraulic channel of an "over-the-wire" dilatation balloon catheter. The openings are sufficiently large to allow the flow of gas therethrough, and yet sufficiently small to prevent the inflation fluid from escaping from the confines of the hydraulic channel.
With respect to the seventh issue, enhanced balloon fragility, polyethylene terephthalate, or PET, is currently being used with increasing frequency to construct the balloon components of low profile catheters. PET is a remarkable material with profound tensile strength. Its use permits the construction of particularly thin-walled balloons capable of withstanding high pressures without rupture. However, this material is particularly fragile and prone to the development of pin-hole tears in response to the usual "wear and tear" that transpires during the course of a routine angioplasty procedure. The development of a pin-hole tear predisposes balloons manufactured with this material to the development of rupture at low pressures. Hence, its use permits the construction of lower profile balloons, in the deflated state, because it can be rendered particularly thin and suitably tolerant to the pressures generated during the course of an angioplasty, and yet its use has been associated with an increased incidence of balloon rupture.
With respect to the eighth issue, reduced balloon profile, a direct relationship exists between the inflation and deflation balloon profiles because dilatation balloons must be constructed with the use of relatively non-compliant materials. Hence, the lower profile catheters commonly carry lower profile balloons. The use of these low profile devices frequently provides incomplete dilatation of the stenosis and thus obligates the use of subsequent catheters, containing larger profile balloons. The performance of a catheter exchange and the use of multiple catheters have been shown to enhance the risk associated with the performance of an angioplasty.
Given the limitations intrinsic to the process of "miniaturization," the fundamental design of the "over-the-wire" catheter was reconfigured with the aim to further reduce the profile and yet circumvent some of the limitations inherent to this process. This effort resulted in the generation of the three additional classes of systems/devices mentioned previously: (1) the "semi-movable" catheter/guidewire composite systems; (2) the "fixed-wire" catheter/guidewire composite systems; and (3) the "balloon-on-a-wire" devices. The design of "semi-movable" and "fixed-wire" systems permits manufacture of these devices with lower profiles relative to "over-the-wire" systems because the guidewire, permanently contained within the confines of these systems, can be rendered lower in profile, relative to the profiles of stand-alone guidewires used in conjunction with "over-the-wire" systems. As in the case of "over-the-wire" systems, these devices contain two separate shaft lumens. The lumen of the guidewire channel in these devices is lower in profile relative to the profile of the guidewire channel contained within "over-the-wire" systems of the prior art. The design of "fixed-wire" systems permits manufacture of these devices with lower profiles relative to "semi-movable" systems because these devices do not contain a central tubular shaft. The guidewires contained within these devices are used to provide column strength for the balloon components. The design of "balloon-on-a-wire" systems permits manufacture of these devices with a lower profile relative to "fixed-wire" systems because the guidewires comprise the shafts of these devices.
Although these designs permit the manufacture of balloon delivery systems with lower profiles relative to "over-the-wire" systems, there exist several functional disadvantages inherent to the designs of these devices. To begin, none of these systems permit separation of the guidewire components from the balloon components and none of these systems accept exchange wires. Hence, the use of these devices obligates removing the guidewire component and sacrificing intraluminal access in the event that a catheter exchange is required.
"Semi-movable devices" provide infinite guidewire rotational mobility. However, they provide limited guidewire/catheter coaxial mobility relative to "over-the-wire" systems. These devices also provide compromised guidewire torque delivery and hence compromised "steerability" because the guidewires contained within these systems are lower in profile relative to "over-the-wire" devices. Finally, the guidewires contained within these devices are more prone to buckling and kinking during introduction across critical stenoses relative to stand-alone guidewires used in conjunction with "over-the-wire" systems.
"Fixed-wire" systems provide less guidewire mobility and deliver less rotational torque relative to "semi-movable" systems. As a result, they provide significantly compromised "steerability" relative to "semi-movable" systems. For example, these devices do not permit coaxial guidewire mobility. Furthermore, they provide only limited rotational mobility. Both of these features relate directly to the fact that the guidewires of these devices are immovably bonded directly to the distal aspects of the balloon catheter components of these systems. This bond permits the guidewire to provide column strength for the balloon and hence constitutes a fundamental component of these devices. And yet, it is because of this bond that the guidewires cannot be advanced or withdrawn in a coaxial direction relative to the catheter components of these devices. It is because of this bond that the guidewires cannot be independently rotated within the confines of these devices. In fact, uni-directional rotation of these guidewires results in progressive wrapping of the balloon components. For this reason, these devices commonly contain torque limiters, as described in U.S. Pat. No. 4,664,113. These torque limiters prevent the operator from over-rotating the guidewire component relative to the catheter component of these systems. Over-rotation of the guidewire can result in stress between the balloon and the guidewire, causing damage to one or both components. Unfortunately, the presence of a torque limiter complicates the angioplasty procedure by requiring the operator to periodically stop the procedure and unwind the guidewire to its "home" position as the operator navigates the catheter through the convoluted arteries of the patient's cardiovascular system.
The presence of this bond, between the guidewire and the distal aspect of the balloon, further compromises the guidewire torque delivery because much of this torque becomes absorbed by the obligate rotation of the balloon component of these devices. In this regard, the balloon can become "hung up" on the luminal surface of a blood vessel. Because rotation of the guidewire requires rotation of the balloon component of these devices, this circumstance dramatically compromises the torque delivery of these devices and hence significantly compromises the "steerability" of these devices within the confines of stenotic lesions.
This practice of bonding the guidewire to the distal aspect of the balloon predisposes to the development of fractures within the segments of the guidewires contained within the balloon components of these devices. These guidewire components are responsible for the delivery of torque to the distal aspect of these devices. Torque delivery is required for directional control. Rotation of these devices subjects these thin and particularly fragile guidewire segments to considerable torsion. The application of excessive rotational force can provoke fractures within the guidewires, a circumstance that commonly transpires when the balloon components become "hung up" within these confines of atheromatous lesions. This circumstance impairs the rotational mobility of the balloon, which, in turn, compromises the rotational mobility and hence directional control of the entire device. This circumstance is commonly met with the application of excessive rotational force. And this response leads to the generation of considerable torsion within the delicate intraluminal guidewire. In the event that the balloon component remains tethered within the confines of the vasculature, then the torsion applied to the guidewire steadily increases with the application of additional rotational torque, until the device eventually fractures.
U.S. Pat. No. 4,715,378 describes a vented "fixed-wire" device. In this patent, a winding passage extending through the bond between the guidewire coil and the distal aspect of the catheter functions as a vent. This device does not appear to have a torque limiter and hence it is prone to over-wrapping with possible rupture of the balloon and fracture of the guidewire.
U.S. Pat. No. 4,793,350 describes another approach for venting a "fixed-wire" device. In this patent, a vent is formed by providing a small space between the guidewire and the distal aspect of the balloon. The catheter and guidewire are fastened together at the distal aspect of the balloon and this device contains a torque limiter.
"Balloon-on-a-wire" systems provide no mobility between the guidewire and the balloon components of these devices. In effect, the balloons on these devices are bonded both proximally and distally to the guidewires. Directional control of these devices is provided by rotation of the entire device. As in the case of "fixed-wire" devices, the practice of immovably bonding the balloons onto the guidewires of "balloon-on-a-wire" devices compromises torque delivery. As in the case of "fixed-wire" devices, this practice predisposes these devices to the development of fractures within the components of the guidewires that span the lumens of these balloons. Tip fracture currently constitutes the fundamental limitation of prior art "balloon-on-a-wire" systems and recently resulted in the FDA recall of the most popular prior art device of this functional class.